The Signaling Pathways Mediated by P2Y Nucleotide Receptors in the Formation and Maintenance of the Skeletal Neuromuscular Junction

Abstract

The motor neuron, the Schwann cell and the muscle cell are highly specialized at the vertebrate skeletal neuromuscular junction (NMJ). The muscle cell surface contains a high local density of acetylcholine (ACh) receptors (AChRs), acetylcholinesterase (AChE) and their interacting macromolecules at the NMJ, forming the postsynaptic specializations. During the early stages of development, the incoming nerve terminal induces the formation of these postsynaptic specializations; the nerve secretes agrin and neuregulin (NRG), which are known to aggregate existing AChRs and to increase the expression of AChR at the synaptic region, respectively. In addition, adenosine 5′-triphosphate (ATP) is stored at the motor nerve terminals and is coreleased with ACh during muscle contraction. Recent evidence suggests that ATP can play a role in forming and maintaining the postsynaptic specializations by activating its corresponding receptors. In particular, one of the nucleotide receptor subtypes, the P2Y₁ receptor, is specifically localized at the NMJs. The gene expression of AChR and AChE is upregulated after the activation of P2Y₁ receptors. Thus, the synaptic ATP together with agrin and NRG can act as a synapse-organizing factor to induce the expression of postsynaptic functional effectors.